a. Field of the Invention
This invention relates to an electrophotographic apparatus. More particularly, the invention is concerned with the electrophotographic apparatus of a construction which uses a photosensitive body of a three-layer structure having an insulating layer on its surface to be repetitively used, and which is effective in preventing non-uniform electric charge remaining on the photosensitive body after its use from adversely affecting the subsequent reproduction operation.
B. Description of the Prior Art
In the above-described type of the electrophotographic apparatus which has so far been used, it is a common practice that a plurality of electric chargers for corona discharge are provided around a photosensitive body in a generally cylindrical or belt-shape (hereinafter referred to simply as "photosensitive layer"), and that latent image formation, image transfer, and related operations are carried out by changing over between a charged state and a stopped state in synchronism with the operation of the apparatus. In such apparatus, the surface potential of the photosensitive layer is in most cases non-uniform immediately after completion of operation of the charger, so that, when the photosensitive layer is left for a long period of time in this state, there inevitably takes place adverse effects, as is well known, such as the non-uniformity of images reproduced during subsequent operations of the apparatus. In order to avoid such an undesirable effect, the electric charge on the photosensitive layer may be discharged by irradiation of light onto the entire surface of the photosensitive layer as has already been known in the xerographic method as disclosed in, for example, U.S. Pat. No. 2,297,691, wherein the photosensitive layer provided on the surface of the electrically conductive body thereof with a photoconductive insulating material is used.
However, such light irradiation onto the photosensitive layer does, by itself, in no way, work to completely remove the electric charge in the electrophotographic method as disclosed in Japanese Patent Publication No. 42-23910, wherein the photosensitive layer of a three-layer structure composed of a substrate, a photoconductive layer, and an insulating layer is used. In this electrophotographic method, an electrostatic latent image may be formed on the surface insulating layer of the photosensitive layer due to the difference in the surface charge density by first uniformly charging the photosensitive layer disposed on the surface of a metal cylinder (hereinafter referred to as "photosensitive drum ") by a primary electric charger (when the photoconductive layer is made of p-type semiconductor, the polarity is negative (-)), then removing the charge by an AC corona discharger impressed with an alternating current voltage with simultaneous irradiation of an original image, and subsequently subjecting the entire surface of the photosensitive layer to a uniform exposure by means of an overall exposure lamp to cause a difference in the surface potential to occur in accordance with the brightness of the original image to be reproduced, thereby obtaining an electrostatic latent image having a high image contrast. Thereafter, toner is adhered onto the latent image by means of a developing device to form a visible image on the photosensitive layer, and the visible image is transferred by a corona charger onto a transfer paper fed to a predetermined position through a paper feed guide, and the paper which has completed the image transfer is separated and sent to an image fixing device. On the other hand, the photosensitive layer which has completed the image transfer is subjected to a cleaning operation to remove the toner remaining on the surface of the photosensitive layer. After this cleaning operation, the electric charge on the photosensitive layer is rendered uniform by means of an AC corona discharger for subsequent use, while it is being subjected to exposure by a small electric bulb. In this type of electrophotographic apparatus, the photosensitive layer is generally in a state of its having been charged extremely non-uniformly right after completion of a series of operations, and leaving the apparatus in this state would cause adverse effects in the subsequent reproduction operation.
To avoid such disadvantage, there has so far been known such a method that the operations of the primary charger and the image transfer charger are stopped, and, while irradiating light onto the photosensitive layer by the overall exposure lamp, the AC corona discharger is operated, and, apart from the process for the image reproduction, the photosensitive drum is rotated to carry out discharge of the photosensitive layer. In this case, however, when the discharge is performed with AC corona, the electric potential of the photosensitive layer tends to assume the negative polarity (-). Also, from the fact that the corona for use in the simultaneous AC discharge and exposure should preferably be of an opposite polarity to that of the primary charge, the AC corona discharger is so designed in some cases that the polarity of the corona is emphatically made negative (-), when the primary charge is made the positive (+) polarity. In this case, if the discharge is carried out by the AC corona without applying the primary charge, the photosensitive layer becomes emphatically charged to the negative (-). It has been discovered as the result of experiments using the AC corona, the polarity of which is emphatically made the negative, the potential of the photosensitive layer reaches as much as -380 volts during a period of only a few revolutions of the photosensitive drum. When the photosensitive layer is made to have such low potential, the potential of the electrostatic latent image will also become uniformly lowered at the time of the subsequent reproduction operation with the consequence that images of good quality cannot be obtained in most cases. Further, when the electric charge on the photosensitive layer is to be made uniform by the electric charger other than the AC corona discharger, there takes place various problems such that, when the AC corona discharger stops its operations, the photosensitive layer which is interposed between the primary charger and the AC corona discharger and which has been subjected to the uniform primary charge passes through the front of the developing device by the rotation of the photosensitive drum, at which time a large quantity of the developing agent adheres onto the surface of the photosensitive layer to contaminate the apparatus.